We propose a PET study using 5-HT1A radiolabelled agonist, C(11)CUMI, to determine whether there is a difference in the density and distribution of 5-HT1A in the high affinity state in the brains of patients with MDD versus controls. We will perform an internal control at 1-5 days after initiation of SSRI to determine whether increased serotonin causes detectable displacement of C(11)CUMI. At 4-8 weeks after initiation of SSRI treatment, we will reimage patients to determine whether there is a change in the density of 5-HT1A in the high affinity state.